Stick a mattress in between and you've saved waste space. *put some pillows on that top shelf, and you've got a reading nook too! the possibilities are endless!
take doors off closet, paint, create shelves, install counter for desk area (look at Menards or Lowe's or Fleet Farm for 'odd sized counters' for sale), maybe mirror part of the back wall for 'light purposes, install outlets for plug-in stuff, slide plastic 3 or 4 drawer bins under counter for storage of office/scrapbook/craft stuff...voila!
Built in Desk in Closet